1. Field of the Invention
The present invention relates to a fluid filled type vibration damping device designed to produce vibration damping action based on flow action of a non-compressible fluid sealed therein, and more particularly to a fluid filled type vibration damping device suitable for use as an automotive engine mount, body mount, difference mount, or the like, for example.
2. Description of the Related Art
In the field of vibration damping devices for installation between components making up a vibration transmission system, it has been proposed to utilize vibration damping action based on the flow action, e.g. the resonance action, of a non-compressible fluid sealed therein. One type of such known device is a fluid filled type vibration damping device comprising: a first mounting member attached one of two components to be linked in a vibration damping fashion; a second mounting member attached to the other of the two components; a main rubber elastic body elastically connecting the first mounting member and the second mounting member which are spaced apart from each other; a pressure-receiving chamber whose wall is partially constituted by the main rubber elastic body and that gives rise of pressure fluctuations during vibration input; an equilibrium chamber whose wall is constituted by a flexible film and that readily permits change in volume; and an orifice passage through which the pressure-receiving chamber and equilibrium chamber communicate with each other. A fluid filled vibration-damping device of this type is effectively used for automotive engine mounts, for example.
In a fluid filled type vibration damping device of this kind, when a large vibrational load is input across the first mounting member and the second mounting member, noise and vibration are sometimes emitted by the vibration damping device. Specifically, in the event that an automotive vehicle employing a fluid filled type vibration damping device of the construction described above as an engine mount should happen to drive over grooved pavement or speed bumps, such noise and vibration can be produced at a level noticeable to passengers in the vehicle.
Such noise and vibration occurs when, during input of impulsive vibration, the flow of the fluid through the orifice passage between the pressure-receiving chamber and the equilibrium chamber cannot keep up, so that a transitory high level of negative pressure occurs within the pressure-receiving chamber. In association with this, gas separates from the filled fluid to form bubbles, a phenomenon known as cavitation. The bubbles maintain generally spherical stable condition from initial occurrence through the growth phase, but during bursting undergo deformation and form tiny explosive jets (microjets). This produces water hammer pressure which is propagated to the first mounting member and the second mounting member, and then transmitted to the vehicle body, and the like. This may cause the problem of noise and vibration mentioned previously.
To address this problem, U.S. Pat. No. 4,697,793, for example proposed a structure wherein a slit is formed in a rubber partition film supported by a partition member which partitions the pressure-receiving chamber from the equilibrium chamber. With this rubber partition film, when the pressure difference between the pressure-receiving chamber and the equilibrium chamber rises above a prescribed level, the rubber partition film undergoes elastic deformation due to the action of the pressure difference, whereby the partition member is induced to open up so that the pressure-receiving chamber and equilibrium chamber communicate. This arrangement makes it possible to eliminate the pressure difference between the pressure-receiving chamber and the equilibrium chamber.
However, with the structure proposed in U.S. Pat. No. 4,697,793, the slit in the rubber partition film opens up to allow communication between the pressure-receiving chamber and the equilibrium chamber through the slit, not only where negative pressure has developed in the pressure-receiving chamber but also where positive pressure has developed. Consequently, any pressure difference between the pressure-receiving chamber and the equilibrium chamber is eliminated by the flow of the fluid through the slit, which can make it difficult to ensure adequate relative pressure fluctuations between the pressure-receiving chamber and the equilibrium chamber. As a result, it becomes difficult to ensure adequate fluid flow level through the orifice passage, posing the risk of difficulty in sufficiently attaining the desired vibration damping action on the part of the orifice passage.
To address this new problem, the Assignee proposed in Japanese Unexamined Patent Publication (JP-A-2003-148548) the use of valve means composed of a rubber elastic body. With this arrangement, when a level of negative pressure higher than the predetermined negative pressure should occur in the pressure-receiving chamber, the orifice passage will be short-circuited by means of a short-circuit passage.
Additional research carried out by the inventors has shown that the fluid filled vibration damping device taught in JP-A-2003-148548 is not yet satisfactory in all instances. Specifically, the valve means taught in JP-A-2003-148548 is composed of a relatively thin rubber elastic body. Due to repeated opening and closing or due to change over time, the rubber elastic body constituting the valve means can experience deformation or deterioration, posing the risk that it will no longer be able to maintain the short-circuit passage opening in the fluid-tightly closed state even where the level of negative pressure in the pressure-receiving chamber is lower than the predetermined negative pressure. Additionally, due to repeated elastic deformation, the rubber elastic body per se may rupture. Also, depending on the required properties, elastic deformation of the valve means composed of the rubber elastic body may not afford sufficient accuracy of recovery to initial shape. Furthermore, with valve means employing a rubber elastic body, variability in dimensions and shape tends to occur readily, making it difficult to attain opening and closing of the valve means with the predetermined negative pressure as threshold, at a high level of accuracy.